The poem begins with a question that sounds almost like a complaint. Why would anyone "carve" speech "laboriously" and fit words together with "curious art", when song, as "one saith", is only a human heart "speaking aloud, undisciplined and free"? That is the familiar idea that poetry should pour out as spontaneous feeling, and Kilmer raises it only to knock it down. The title names the man being addressed, a Jesuit priest ("S. J."), and the poem treats his painstaking work with language as the whole point of him.
The answer comes in line 5: "Nay, God be praised, Who fixed thy task for thee!" Discipline is not a loss of freedom here. It is an assignment, a vocation. Kilmer builds this idea from the vocabulary of the workshop ("carve", "match and blend", "craftsman") and then lifts it into the sanctuary. The poet's "phrased paten" is the plate that holds the communion bread, so the finished poem becomes a vessel on which "the Splendour" can rest. The other poets "traffic in Apollo's mart", a marketplace of buying and selling, while Hopkins is "set apart" and works at an altar. The phrase "austere, ecstatic craftsman" holds the poem's paradox in four words: strict self-control and overwhelming joy belong together.
The form backs up the argument. This is a sonnet in iambic pentameter, and its rhyme scheme is strict: *abba abba* in the octave, then alternating *cdcdcd* in the sestet. Look at how the octave works. "Laboriously", "free", "thee" and "be" all share one sound, so the opening lines are pulled back again and again to the same note, as though the poem were accepting the constraint it praises. The long, stretched word "laboriously" also makes the reader do a little work, and the line pays for it. The diction ("didst", "saith", "knewest", "hast") is deliberately old and prayer-like, which gives the poem the tone of a hymn.
The turn at line 9 swings from "thou" to "we", and the contrast is humbling: "carelessly we throw a rhyme to God, / Singing His praise when other songs are done." The verbs are casual, even rude. We toss a rhyme as if it were a coin, and we offer God the leftovers. Against that, Hopkins's care looks like real reverence. Then the question: "Losing thyself, what is it thou hast won?" It points toward mystics like Teresa, whose "paths" he followed, and toward a paradox the poem believes in, that you find yourself by giving yourself away. The poem implies that self-expression is the smaller ambition.
The last two lines answer in exclamations rather than argument, and the poem becomes pure image. "O bleeding feet, with peace and glory shod!" joins pain to peace in one picture, and "shod" quietly picks up the walking of "paths Teresa trod", so that the feet are both wounded and dressed in glory. The final line, "O happy moth, that flew into the Sun!", is the boldest. A moth drawn to light is usually a symbol of foolishness and self-destruction, yet Kilmer calls it "happy". The capital S makes this a sun worth burning for, and some readers will also hear a pun on "Son", though the poem never says so. The poem ends on brightness and gives no explanation, as if the argument had been left behind.
What gives the poem its interest is that it is a tribute and an argument at once. One poet defends another's difficulty by claiming that effort, restraint and even self-loss can be the highest form of devotion. There is a slight irony in it too. Kilmer praises a craftsman whose reputation rests on dense, knotty language, yet his own sonnet moves smoothly and conventionally. It can describe the sacrifice of the moth, but it keeps its own wings carefully folded. That gap may be modesty, or it may be the poem's one unintended admission that admiring such burning is easier than doing it.